14.4 Gas Mixtures and Molecular Speeds
14.4 Gas Mixtures and Molecular Speeds
Lesson Objectives
- Use Dalton’s law and mole fraction to calculate the partial pressure of a gas in a mixture.
- Calculate the pressure of a gas that has been collected by water displacement in order to determine the volume
of the dry gas. - Define diffusion and effusion.
- Use Graham’s law to calculate the velocity ratios of two gases based on their molar masses.
Lesson Vocabulary
- Dalton’s law of partial pressures
- diffusion
- effusion
- mole fraction
- partial pressure
Check Your Understanding
Recalling Prior Knowledge
- Do gas molecules interact with one another as they exert pressure?
- How is the vapor pressure of water related to its temperature?
- How is kinetic energy related to the mass and velocity of a moving object?
The gas laws that have been introduced so far have applied to a pure gas. In this lesson, you will learn about Dalton’s
Law, which is important for dealing with mixtures of gases. In addition, the velocities of gas molecules will be
studied by use of Graham’s Law.
Dalton’s Law
Gas pressure results from collisions between gas particles and the walls of their container. If more gas is added to a
rigid container, the gas pressure increases. This is true whether the newly added gas is the same as the original gas or
a different one; the identities of the gases do not matter. John Dalton, the English chemist who proposed the atomic
theory, also studied mixtures of gases. He found that each gas in a mixture exerts a pressure independently of every
other gas in the mixture. For example, our atmosphere is composed of about 78% nitrogen and 21% oxygen, with
smaller amounts of several other gases making up the rest. Since nitrogen makes up 78% of the gas particles in a